1. Field of the Invention
The present invention relates to a infrared detectors, specifically infrared detectors operable across a wide temperature range having either improved performance at the cryogenic temperatures or useful performance at near room temperature operation.
2. Description of the Prior Art
Copending applications have been filed by common inventors for related subject matter. Application Serial No. 07/092,736, entitled "Bandgas Radiation Detector" was filed on Aug. 5, 1987 by Dayton Eden and William Case. Copending application Serial No. 06/901,071 entitled "Uncooled Infrared Detector" was filed on Jul. 14, 1986.
The detection of infrared energy particularly from targets near or around room temperature, in the midwave and longwave infrared range using focal plane detectors that can operate warmer than 200.degree. K. is extremely desirable in avionic and space based applications.
The best detectors in the infrared range, have in the past required significant cooling capability in the liquid nitrogen temperature range of less than 100.degree. K. so that they might operate at peak performance. Their detector performance radically decreased as they are operated at closer to room temperatures, which detectors include uncooled conventional photoconductive or photovoltaic HgCdTe detectors, bolometers, and pyroelectric arrays do not achieve the same detectivity as the cryogenic detectors. Their performance is poorer by at least two orders of magnitude due to limitations in the detection mechanisms and noise sources when operating near room temperature.
Any object, animate or inanimate emits infrared energy. The atmosphere above the earth is transparent in two spectral regions to the radiation in the mid-wavelength infrared (MWIR) and the longwave infrared wavelength (LWIR) bands. A body of temperature 300.degree. K., again mechanical or human, emits the peak of its radiation in the longwave infrared (LWIR) band. The longwave infrared range of detection is in the 8 micron (.mu.) to 12 micron (.mu.) range. The midwave infrared is in the 3 micron (.mu.) to 5.3 micron (.mu.) range.
Conventional state-of-the-art photosensitive infrared detectors are typically fabricated out of 10 .mu. (micron) thick bulk or epitaxial layers to maximize the absorption efficiency. Detectors are then delineated by appropriate means in these layers. Typically for detectors which are operating in a "detector limited" performance region, the largest contributing noise source comes from thermal generation-recombination, noise (thermal g-r noise), which results from carrier density fluctuations. Detectivity of a detector is described by using a combination of response and noise measurements analytically formulated to define a function called D* (D-star).
The D* associated with a thermal g-r limited detector goes as proportional to the inverse square root of the volume of the detector. Thus, if one can reduce the detector volume while maintaining a high absorption and providing high quantum efficiency, then the detectors performance can be improved. This can be done by patterning the active detector material into an array type structure with resonant characteristics. The volume of the detector material in this resonant detector is thereby reduced not only because the active area is reduced but also because it can be made substantially thinner while maintaining a high absorption cross section and thus high quantum efficiency.
Advances in lithography have made it possible to apply and scale these microwave concepts to the infrared region. The performance improvements which result from applying microwave concepts to the infrared provides D* values improved over the state of the art detectors operating at cryogenic temperatures.
Conversely, the operating temperature of a resonant structure can be raised significantly toward the room temperature level to provide a D* equivalent to the current state of art detectors which would be operating at 77.degree. K.
The problem to be solved therefore, is the problem of producing an array of infrared detectors that are operable in the 3 micron (.mu.) to 5 micron (.mu.) midwave infrared (MWIR) spectral region or 8 micron (.mu.) to 12 micron (.mu.) longwave infrared region (LWIR) that are; (1) responsive with low noise at higher operating temperatures; (2) require less cooling equipment to achieve performance as found in the prior art. Simultaneously, it would be advantageous to have an infrared detector operable effectively in a wide temperature range.
The following articles include subject matter which may be related to the technology disclosed herein.
1. D. B. Rutledge and S. E. Schwarz, "Planar Multimode Detector Arrays for Infrared and Milimeter-Wave Applications", IEEE Journal of Quantum Electronics, Vol. QE-17, No. 3, March, 1981, pp. 407-413.
2. J. N. Schulman and T. C. McGill, "The CdTe/HgTe Superlattice: Proposal for New Infrared Material", Appl. Phys. Lett. 34 (10), May 15, 1979, pp. 663-665.
3. J. P. Faurie, S. Sivananthan, M. Boukersche and J. Reno, "Molecular Beam Epitaxial Growth of High Quality HgTe and Hg (1-x) Cd(x) Te Onto GaAs (001) Substrates", Appl. Phys. Left. 45 (12), Dec. 15, 1984, pp. 1307-1309.
4. D. L. Smith, D. K. Arch, R. A. Wood, and M. Walter Scott, "HgCdTe Heterojunction Contact Photoconductor", Appl. Phys. Lett. 45 (1), Jul. 1, 1984, pp. 83-85.
5. D. L. Smith, "Theory of Generation-Recombination Noise in Intrinsic Photoconductors", J. Appl. Phys. 53 (10), Oct., 1982, pp. 7051-7060.
A solution to these problems is presented by the preferred embodiment of this invention which, while applicable particularly to the longwave infrared range, would also function in the midwave infrared spectral and longer wavelength infrared spectral ranges.